1. Field of the Invention
The present invention relates to white balance adjusting apparatus, and more particularly to a white balance adjusting apparatus in an image sensing apparatus such as a color video camera for automatically adjusting white balance to correct the wavelength distribution of light differing by each light source, according to the luminance information signal and the color information signal within the image sensing signal obtained from an image sensing device.
2. Description of the Background Art
In taking an object using an image sensing apparatus such as a color video camera, the wavelength distribution of light illuminating the object from a light source differs by the type of the light source. For example, the blue components are intensive in light from a light source of relatively high temperature, whereas the red components are intensive in light from a light source of relatively low temperature. It is therefore necessary to correct the wavelength distribution of each light source in order to properly reproduce the color tone of the object itself illuminated with light of the light source on the screen of a color monitor television receiver. This correction is generally called white balance adjustment, where the gain of each color signal is adjusted so that the ratio of the amplitudes of the three primary color signals of red (hereinafter referred to as R), blue (hereinafter referred to as B), and green (hereinafter referred to as G) is 1:1:1.
In conventional image sensing apparatus, the detection of the three primary color signals R, G, and B is carried out according to light around the image sensing apparatus using a sensor provided for each color. However, white balance could not be adjusted correctly with such image sensing apparatus when the light source around the image sensing apparatus (for example, fluorescent light) differs from the light source illuminating the object (for example the sun), as in the case where an outdoor scene is taken from inside a room.
Recently, a method called TTL (through-the-lens) is proposed in which white balance adjustment is carried out, without providing separate sensors, according to color difference signals R-Y and B-Y within the image sensing signal obtained from an image sensing device. Such a method is disclosed in Japanese Patent Laying-Open No. 62-35792, for example. This method is based on the consideration that the object taken by an image sensing apparatus has various color area distribution (hereinafter referred to as the color distribution) and if this color distribution is averaged over a sufficient long time, the color components cancel each other to result in each color signal of "0", which is equivalent to taking a completely white picture. By controlling the gains of respective color signals so that the values resulting from integration of color difference signals R-Y and B-Y over one field period, for example, become 0, to correct the offset of the color tone due to wavelength distribution of light of the light source is corrected.
FIG. 1 is a block diagram showing an example of a conventional white balance adjusting apparatus by the TTL method. Referring to FIG. 1, light from an object (not shown) enters an image sensing device 2 formed of a CCD via a lens 1. The incident light is converted by a photoelectric device into an electric signal and provided to a color separating circuit 3. Color separating circuit 3 extracts the three primary color signals of R, G, and B from this electric signal. The extracted G signal is directly provided to a camera processing and matrix circuit 6. The R signal and B signal are provided to camera processing and matrix circuit 6 via a gain variable gain R amplifying circuit 4 and a B amplifying circuit 5, respectively. Camera processing and matrix circuit 6 creates a luminance signal Y and color difference signals R-Y and B-Y according to the three primary color signals of G, R, and B. The outputs are provided to a video circuit 7 where luminance signal Y and color difference signals R-Y and B-Y are subjected to the well-known process to create a recordable video signal. This recordable video signal is provided to a video recording circuit not shown.
The two color difference signals R-Y and B-Y are applied to integrating circuits 18 and 17, respectively, to be integrated over a sufficient long time, for example over 1 field period of a video signal. The values resulting from the integration are provided to gain control circuits 13 and 14. Gain control circuits 13 and 14 control the variable gains of B amplifying circuit 5 and R amplifying circuit 4 so that the values resulting from integration each becomes 0. This results in the amplitude ratio of 1:1:1 of the three primary color signals G, R, and B to adjust white balance.
In a conventional white balance adjusting apparatus of FIG. 1, there are some cases where at least one of the outputs of a plurality of light reception portions (not shown) provided for respective colors of R, G and B and constituting image sensing device 2 become saturated. Typically, saturation occurs when there is an object of significantly high luminance in a portion of the image sensed picture, such as in the case where a light source such as the sun is taken. Consequently, the amplitude ratio of the three primary color signals R, G, and B provided from color separating circuit 3 is not proportional to the actual R, G and B components included in the light source. Therefore, there is a problem when the white balance is offset in a direction not associated with the actual light source color temperature resulting from the white balance being adjusted according to improperly proportioned three primary color signals R, G, and B.
The conventional white balance adjusting apparatus of FIG. 1 corrects the irregularity of the wavelength distribution due to light of the light source, based on the consideration that colors cancel each other so that the reproduced picture can approximate a substantially white picture if the various color distributions of the object itself are averaged over a long period. This method is inaccurate when white balance regarding the object itself can not be achieved because the reproduced picture can not approximate a white picture even if the color distributions of the object included in the entire picture are averaged. This arises when the area ratio of the three primary colors within the picture is not equal, that is to say, when the color distribution is not even, such as in the case where green lawn or a blue sky occupies a large area of the picture, or in the case where a human object wearing a red sweater is taken in a close-up manner. If the above mentioned white balance adjustment is applied to such an unbalanced state of white balance, the gain will be controlled so as to cancel the intensive color. In the case of a close-up of a person wearing a red sweater, white balance will be unnecessarily intense in blue which is the complementary color of red, resulting in the color of the object being improperly reproduced on the reproduced screen.
Particularly, blue sky will be located in the upper end of an image sensed picture when taken outdoors. White balance of the entire picture will be intense in the side of the complementary color of blue. Because there are many cases where the blue sky is located at the upper end of a picture, it is desirable to consider an effective solution.
When the luminance of an object is extremely low, each level of the three primary color signals of R, G, and B will be reduced to aggravate the S/N ratio. This will unbalance the R, G, and B signal levels and generate color difference signals even though the actual object is black, i.e. achromatic color. In the conventional white balance adjusting apparatus of FIG. 1, the gains of signals R and B will be varied to cancel these inaccurate color difference signals, whereby white balance is intense in the complementary color side.